Chromizing over cathodic arc coating

ABSTRACT

The present invention provides a Cr-rich cathodic arc coating, an article in turbine blade coated with the chromizing over cathodic arc coating, and a method to produce the coating thereof. The Cr-rich cathodic arc coating in the present invention comprises a cathodic arc coating and a diffusion coating deposited atop the cathodic arc coating to enforce hot corrosion resistance. The hardware coated with the chromizing over cathodic arc coating in the present invention is reinforced with superior-hot corrosion resistance. The present invention further provides a novel method for producing the chromizing over cathodic arc coating by re-sequencing coating deposition order. The method in the present invention is efficient and cost-reducing by eliminating some operations, e.g., DHT and peening, between the cathodic arc coating and the diffusion coating. The hot corrosion resistance in the present invention results from the high Cr content in the surface of the coating.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/103,761 filed on Jun. 10, 2016 which is a National Phase Applicationof Patent Application PCT/US2014/066277 filed on Nov. 19, 2014, whichclaims the benefit of and priority to U.S. Provisional PatentApplication No. 61/914,222, filed Dec. 10, 2013, the contents each ofwhich are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

This invention relates to a Cr-rich cathodic arc coating which providessuperior hot corrosion resistance, an article with such Cr-rich cathodicarc coating, and a method to produce the Cr-rich cathodic arc coating byre-sequencing coating deposition order.

BACKGROUND

High-temperature corrosion (hot corrosion) is a mechanism of corrosionthat takes place in gas turbines, diesel engines, furnaces or othermachinery coming in contact with hot gas containing certaincontaminants. The hardware of such machinery, e.g., a turbine blade, iscontinuously threatened by extreme hot corrosion. In such region of gasturbines or engines, subsequent coating may be applied to provide asuperior hot-corrosion resistance to gas turbines.

Many factors in such coating systems, both microstructural (grain size,distribution of second phase precipitates, etc.) and compositional(chemical homogeneity, bulk content of refractory metal, Cr, Co, Al, Y,Hf, etc.), can influence hot corrosion resistance performance. Amongthose, chromium (Cr) content is a key contributor to hot corrosionresistance. As such, it has been common to apply a secondary coatingunder the high chromium cathodic arc coating to increase hot-corrosionresistance.

Nevertheless, the conventional combined coating process requiresconsiderable additional cost and causes significant loss in coatingthickness due to aggressive grit blast and spallation. As such, there isa need to provide a coating which provides High-temperature corrosion(hot corrosion) resistance without the cost and loss associated withconventional methods.

SUMMARY OF THE INVENTION

The present invention provides a novel Cr-rich cathodic arc coating, anarticle coated with the Cr-rich cathodic arc coating and a method toproduce the same.

In one embodiment, a chromium-rich cathodic arc coating comprising;

a MCrAlY, wherein M is a metal alloy comprising nickel, cobalt, iron, ora combination thereof, on a substrate; and

a diffused chromide coating atop the MCrAlY,

wherein a surface of the diffused chromide coating has high content ofchromium.

In certain embodiments, the surface of the diffused chromide coatingatop the MCrAlY contains chromium content from about 20% to about 50% byweight.

In other embodiment, the MCrAlY contains chromium content between about25% and 50%, between about 25% and 40% or between about 25% and 35% byweight based on the weight of the MCrAlY.

In another embodiment, the MCrAlY comprises Co, Cr, Al, Hf, Y, orcombinations thereof and the diffused chromide coating comprises a Crrich phase.

In still another embodiment, the MCrAlY further comprises Si. In certainembodiments, the MCrAlY comprises Si in an amount of about 0.10-0.70% byweight based on the weight of the MCrAlY. In other embodiments, theMCrAlY comprises Si in an amount of about 0.15-0.65% by weight based onthe weight of the MCrAlY.

In particular embodiment, the chromium-rich cathodic arc coating isresistant to hot corrosion at a temperature between about 1200° F. and1600° F.

In another particular embodiment, the chromium-rich cathodic arc coatingis resistant to stress corrosion or to low cycle fatigue.

Still in another certain embodiments, the substrate comprises a bladeroot, internal surface of turbine blade. In particular embodiments, thesubstrate can be the external surface of a blade, in whole or in part.

According to the present invention, an article blade comprising:

a substrate;

a MCrAlY on a substrate, wherein M is a metal alloy comprising nickel,cobalt, iron, or a combination thereof; and

a diffused chromide coating atop the MCrAlY.

wherein a surface of the diffused chromide coating atop the MCrAlY hashigh content of chromium.

In another embodiment, the MCrAlY comprises Co, Cr, Al, Hf, Y, orcombinations thereof and the diffused chromide coating comprises a Crrich phase.

In still another embodiment, the MCrAlY further comprises Si. In certainembodiments, the MCrAlY comprises Si in an amount of about 0.10-0.70% byweight based on the weight of the MCrAlY. In other embodiments, theMCrAlY comprises Si in an amount of about 0.15-0.65% by weight based onthe weight of the MCrAlY.

According to the present invention, a method of producing achromium-rich cathodic arc coating comprises steps of:

applying a MCrAlY on a substrate, wherein M is a metal alloy comprisingnickel, cobalt, iron, or a combination thereof; and

applying a diffused chromide coating atop the MCrAlY;

wherein a surface of the diffused chromide coating atop the MCrAlY hashigh content of chromium.

In some embodiments, the diffused chromide coating contains chromiumcontent upwards of 20% to about 50% by weight.

In other embodiment, the MCrAlY contains chromium content between about25% and 50%, between about 25% and 40% or between about 25% and 35% byweight based on the weight of the MCrAlY.

In particular embodiments, the method of applying the MCrAlY comprisescathodic arc physical vapor deposition. In another particularembodiments, the method of applying the diffused chromide coating atopthe MCrAlY comprises pack chromizing, slurry chromizing, vapor diffusioncoating, or gas diffusion coating. In certain embodiments, the diffusedchromide coating is applied at an elevated temperature from about1800-2200° F., from about 1900-2100° F., or from about 1925-2000° F.

BRIEF DESCRIPTION OF THE DRAWINGS

So that those skilled in the art to which the subject disclosureappertains will readily understand how to make and use the devices andmethods of the subject disclosure without undue experimentation,preferred embodiments thereof will be described in detail herein belowwith reference to certain figures, wherein:

FIG. 1 shows an optical micrographic view of a conventional diffusioncoating, demonstrating the presence of oxide and nitride inclusions onthe top portion.

FIG. 2 shows optical microscopic views of coating on a substrate; a)MCrAlY only; b) diffused chromide coating/MCrAlY with full preparationof cathodic arc coating; c) diffused chromide coating/MCrAlY with nopreparation of cathodic arc coating; d) MCrAlY/diffused chromidecoating.

FIG. 3 shows a SEM image and a Cr X-ray map showing a formation of anelevated Cr-rich phase on the surface of the Cr-rich cathodic arccoating.

FIG. 4 is a plot of maximum stress vs. cycles showing failure point toLCF test with each coating specimen.

FIG. 5 shows macroscopic images of sample substrates, which are coatedwith in each MCrAlY only; MCrAlY/diffused chromide coating FP;MCrAlY/diffused chromide coating NP, view after: a: 21 hours; b: 315hours; c: 415 hours; d: 737 hours; e: 980 hours of hot corrosion testingat 1350° F.

FIG. 6 shows microscopic images of sample substrates after 980 hours ofhot corrosion testing at 1350° F.; with MCrAlY/diffused chromidecoating; MCrAlY/diffused chromide coating; MCrAlY/diffused chromidecoating, coating specimens.

DETAILED DESCRIPTION

The present invention provides a chromium-rich cathodic arc coating, anarticle coated with the chromizing over cathodic arc coating, and amethod to produce thereof.

Definition

As used herein the term “chromium-rich” refers to an elevated chromecomposition in coating material exhibiting excellent hot corrosionresistance. In certain embodiments, the surface of “chromium-rich”coating contains upwards of 40% of chromium content by weight based onthe weight of coating.

As used herein the term “high-content” refers to a coating materialhaving excessive amount of composition than a conventional coating. Incertain embodiments, the term “high-content” chromium refers to chromiumcontent in coating upwards of 40% by weight. In other embodiments,“high-content” chromium exhibits excellent hot corrosion resistance.

As used herein, the term “stress corrosion” refers to a defect or afailure due to tensile stress from environment.

As used herein, the term “low cycle fatigue” refers to a stress that iscycled until the damage to plastic or coating occurs at the low cyclenumber, generally 10³˜10⁵ cycles.

As used herein, the term “chromium rich phase” refers to area where thechromium content of an alloy substrate, coating or other metallicsurface has been modified by a diffusion process that causes an increasein chromium content. In some embodiments, the chromium content of thediffused chromide coating is a high-content of chromium. In otherembodiments, the chromium content of the diffused chromide coating isgreater than about 10% chromium chromium, greater than about 15%chromium chromium, greater than about 20% chromium chromium, greaterthan about 25% chromium chromium, greater than about 30% chromiumchromium, greater than about 35% chromium chromium, greater than about40% chromium chromium, greater than about 45% chromium chromium, orgreater than about 50% chromium chromium. In still other embodiments,the chromium rich phase has a specific depth. In particular embodiments,the depth of the chromium rich phase is from about 0.1 to about 25thousandths of an inch, or from about from 0.2 to 10 thousandths of aninch.

Hot Corrosion and Cr Content

During the operation of gas turbine, certain chemicals, such as sulfurspecies, are released at elevated operating temperature and react withenvironmental salts. As consequence, the corrosive contaminants depositon the hot hardwares of gas turbine and accelerate corrosion of metallicsubstrates. This is generally referred to as “hot corrosion”.

Even though the temperature of hot corrosion attack depends on the alloycomposition, hot corrosion attack generally occurs at high temperaturein the range of 1470° F.˜1830° F. and low temperature in the range of1200° F.˜1380° F. The mechanism of hot corrosions are considered to bedifferent between high temperature range and low temperature range, butCr content is believed to be a key factor in alloy composition forenhancing the hot corrosion resistance in both temperature ranges. Forexample, a chromium content of no less than 37% is effective to providehot corrosion resistance at both high and low temperature ranges,compared to the coating containing about 20% Cr.

The high Cr content in the alloy or coating results in formation of thinprotective Cr-rich oxide scales on the surface. This Cr-rich oxide scaleprevents the corrosive chemical contaminants from depositing on thehardware in gas turbine. Such protective oxide scale is also illustratedin FIGS. 5 and 6.

Cathodic Arc Coating: MCrAlY

A cathodic arc coating or deposition is a type of physical vapordeposition. The cathodic arc coating employs an electric arc to vaporizeand ionize coating material, e.g., metal, ceramic, and composites, fromthe cathode. Then, the vaporized material forms a thin layer on thesurface of the substrate. Many of metal based coating materials areapplied using cathodic arc coating, including titanium, zirconium,chromium or alloys thereof.

In certain embodiments of the invention, MCrAlY is applied usingcathodic arc coating. The MCrAlY is applied by, without limitation,using the cathodic arc physical vapor deposition method. According tothe invention, the MCrAlY comprises chromium, cobalt, aluminum, hafnium,yttrium, or a combination thereof. In specific embodiments, the MCrAlYcontains, without limitation, contains chromium content between about20% to about 50% by weight. In other embodiment, the MCrAlY containschromium content between about 25% and 50%, between about 25% and 40% orbetween about 25% and 35% by weight based on the weight of the MCrAlY.

In still other embodiments, the McrAlY further comprises Si. In certainembodiments, the McrAlY comprises Si in an amount of about 0.10-0.70% byweight based on the weight of the McrAlY. In other embodiments, theMcrAlY comprises Si in an amount of about 0.15-0.65% by weight based onthe weight of the McrAlY.

Diffusion Coating: Diffused Chromide Coating

Diffusion coating is a type of coating process by allowing alloy orcoating materials diffuse to under-layer at high temperature. Thediffused coating material imparts alternative chemical compositions tobase materials in order to improve physical or mechanical properties.Based on the different elements in composition of material, diffusioncoating can be aluminizing, chromizing, molybdenization, manganizing,chrome calorization, and chrome-titanium plating. Diffusion of coatingmaterial is available from solid, gas, vapor, or liquid phase. Becausethe diffusion coating performs at high temperature, e.g. 1800° F., someelements of under-lying base or other coating may diffuse during theprocess.

In certain embodiments in the invention, the diffused chromide coatingis applied by diffusion coating process. In particular embodiments, thediffused chromide coating is applied by pack chromizing, slurrychromizing, vapor diffusion coating, or gas diffusion coating. Incertain embodiments, the diffused chromide coating is applied at anelevated temperature from about 1800-2200° F., from about 1900-2100° F.,or from about 1925-2000° F.

In certain embodiments of the invention, high content of Cr from theMCrAlY of base coating diffuses during subsequent diffusion coatingprocess. For example, the cathodic arc coating may contain a highcontent of Cr initially which, during the diffusion coating atop theMCrAlY, diffuses toward the surface. As consequence, the surfacecontains higher Cr content than MCrAlY alone or a coating of MCrAlY atopa diffused chromide coating surface. As evidence, FIG. 3 shows a nearlycontinuous chromium-rich single phase at the surface of diffusedchromide coating from the diffusion coating deposition.

Methods

The present invention provides a novel method for producingchromium-rich cathodic arc coating diffused chromide coating/MCrAlY bydepositing the diffused chromide coating surface atop the cathodic arccoating. By coating the substrate in this method, the coating process inthe present invention does not require subsequent post-treatment, suchas grit blast, diffusion heat treatment (DHT), or peening operation, onthe underlying cathodic arc coating MCrAlY, thereby reducing cost andtime significantly. Furthermore, the chromium-rich cathodic arc coatingby this method acquires highly improved hot corrosion resistance.Meanwhile, no negative influence during low cycle fatigue (LCF) test orstrain to crack (StC) test has been identified compared to theconventional MCrAlY coating, MCrAlY/diffused chromide coating, or thefully treated diffused chromide coating/MCrAlY.

According to the present invention, a method of producing achromium-rich cathodic arc coating comprising steps of:

-   -   a) applying MCrAlY on a substrate, wherein M is a metal alloy        comprising nickel, cobalt, iron, or a combination thereof;    -   b) applying diffused chromide coating atop MCrAlY;    -   wherein a surface of diffused chromide coating atop MCrAlY has        high content of chromium.

In certain embodiments, the surface of the diffused chromide coatingcontains chromium content of about 20% to about 50% by weight. In otherembodiment, the MCrAlY contains chromium content between about 25% and50%, between about 25% and 40% or between about 25% and 35% by weightbased on the weight of the MCrAlY.

In another embodiment, the MCrAlY comprises CO, Cr, Al, Hf, Y, orcombinations thereof and the diffused chromide coating comprises a Crrich phase.

In still another embodiment, the McrAlY further comprises Si. In certainembodiments, the McrAlY comprises Si in an amount of about 0.10-0.70% byweight based on the weight of the McrAlY. In other embodiments, theMcrAlY comprises Si in an amount of about 0.15-0.65% by weight based onthe weight of the McrAlY.

In particular embodiments, the method of applying the MCrAlY comprisescathodic arc physical vapor deposition. In another particularembodiments, the method of applying the diffused chromide coating atopthe MCrAlY comprises pack chromizing, slurry chromizing, vapor diffusioncoating, or gas diffusion coating.

In certain embodiments, the diffused chromide coating is applied at anelevated temperature from about 1800-2200° F., from about 1900-2100° F.,or from about 1925-2000° F.

An Article with the Chromium-Rich Coating

The article coated with the chromium-rich cathodic arc coating in thepresent invention is reinforced with superior hot-corrosion resistance.The article taking advantages from the chromium-rich cathodic arccoating may be a part of gas turbine engine of aircraft.

According to the present invention, an article in a turbine bladecomprises:

-   -   a substrate;    -   a MCrAlY on the substrate, wherein M is a metal alloy comprising        nickel, cobalt, iron, or a combination thereof; and    -   a diffused chromide coating atop the MCrAlY;

wherein a surface of the diffused chromide coating atop the MCrAlY hashigh content of chromium.

In one embodiment, the substrate comprises a blade root, the internalsurface of turbine blade, or the external surface of a turbine blade. Inanother embodiment, the MCrAlY comprises CO, Cr, Al, Hf, Y, orcombinations thereof and the diffused chromide coating comprises a Crrich phase.

In particular embodiments, the method of applying the MCrAlY comprisescathodic arc physical vapor deposition. In another particularembodiments, the method of applying the diffused chromide coating atopthe MCrAlY comprises pack chromizing, slurry chromizing, vapor diffusioncoating, or gas diffusion coating.

In certain embodiments, the diffused chromide coating is applied at anelevated temperature from about 1800-2200° F., from about 1900-2100° F.,or from about 1925-2000° F.

A better understanding of the present invention may be obtained throughthe following examples which are set forth to illustrate, but are not tobe construed as limiting the present invention.

Example 1: Operation to Produce Coating Specimens

The processes to deposit the various coating specimens in the presentapplication are summarized in Table 1.

TABLE 1 Definition of the processes to deposit the coatings specimensOperations cathodic diffusion heavy grit cathodic coating specimen arccoating DHT peening coating blast arc coating DHT peening MCrAIY 1 2 3MCrAIY/diffused 1 2 3 4 5 chromide coating diffused chromide 1 2 3 4coating/MCrAIY (FP) diffused chromide 1 2 coating/MCrAIY (NP)

A conventional cathodic arc coating MCrAlY is used as a baseline in thehot corrosion and LCF tests.

The coating specimen MCrAIY/Diffused chromide coating is processed bythe conventional method by depositing sequentially a diffusion coating,heavy grip blast to remove the coating surface, and a cathodic arccoating, followed by DHT and peening operation. The MCrAIY/Diffusedchromide coating specimen is also used as a baseline in the hotcorrosion and LCF tests.

The coating specimen diffused chromide coating/MCrAIY (FP) and thecoating specimen diffused chromide coating/MCrAIY (NP) are produced bythe procedure in the invention. As shown in Table 1, the specimendiffused chromide coating/MCrAIY (FP) is “fully prepared (FP)” Cr-richcathodic arc coating with DHT and peening operation inbetween cathodicarc deposition and a diffusion coating, and namely FP Cr-rich cathodicarc coating. The coating specimen diffused chromide coating/MCrAIY (NP)is “not prepared (NP)” with such operation after cathodic arcdeposition, namely NP Cr-rich cathodic arc coating.

Example 2: Formation of Chromizing Over Cathodic Arc Coating

FIG. 2 shows optical microscopic views of each coating specimen.

Nominal coating thicknesses are between about 2.5 mil and 3 mil.

MCrAIY and MCrAIY/diffused chromide coating specimens are observed tohave consistent microstructures as previously predicted.

In contrast, diffused chromide coating/MCrAIY (FP) and diffused chromidecoating/MCrAIY (NP) specimen coatings contain oxides/nitridesinclusions. The majority of these coatings was found to be single phase,likely 7-Ni FCC solid solution, due to Cr enrichment resulting from thediffusion coating deposition. Angular Al-rich phases, like nitrides,resulting from the diffusion coating process, are visible in thesecoatings as well.

As shown in the SEM image and associated Cr x-ray map in FIG. 3, thedeposition of diffused chromide coating atop MCrAlY on a scrap bladeresults in the formation of a nearly continuous highly elevated Cr-richphase at the surface.

Example 3: Low Cycle Fatigue (LCF) Test Result

LCF tests were performed with the coating specimens at the temperatureof 1200° F.

FIG. 4 shows a plot of maximum stress value vs. cycles to failure foreach LCF testing. R value for this plot is 0.5.

Maximum stress value for each specimen was investigated within the rangeof 120 ksi-140 ksi. At a maximum stress of 120 ksi, no significantdifferences were observed between each coating specimen; failuregenerally occurred at approximately 100,000 cycles. At 140 ksi maximumstress, MCrAIY and MCrAIY/diffused chromide coating specimens failedwithin the first 100 cycles. diffused chromide coating/MCrAIY (FP) anddiffused chromide coating/MCrAIY (NP) specimens failed at greater than1000 cycles. There is no distinction between the test results ofdiffused chromide coating/MCrAIY (FP) and diffused chromidecoating/MCrAIY (NP).

In conclusion, there is no indication that LCF resistance is associatedwith the preparation process, e.g., DHT and peening, to MCrAIY prior todiffused chromide coating. Furthermore, there was no observation of anyLCF or strain to crack debit with the preparation process described.

Example 4: Strain to Crack Test at Different Operating Temperature

The strain to crack (StC) test were conducted with the coating specimensMCrAIY/diffused chromide coating, diffused chromide coating/MCrAIY (FP)and diffused chromide coating/MCrAIY (NP). The StC tests were performedat 600° F., 800° F., 1000° F., and 1200° F. and the test results arepresented in Table 2.

TABLE 2 Coating strain to crack at various temperatures Coating specimenTemperature (° F.) Strain to Crack MCrAlYI diffused 600 0.81 chromidecoating diffused chromide coating/ 600 No MCrAlY (FP) Cracking diffusedchromide coating/ 600 No MCrAlY (NP) Cracking MCrAlYI diffused 800 0.85chromide coating diffused chromide coating/ 800 No MCrAlY (FP) Crackingdiffused chromide coating/ 800 No MCrAlY (NP) Cracking MCrAlYI diffused1000 0.88 chromide coating diffused chromide coating/ 1000 No MCrAlY(FP) Cracking diffused chromide coating/ 1000 No MCrAlY (NP) CrackingMCrAlYI diffused 1200 No chromide coating Cracking diffused chromidecoating/ 1200 No MCrAlY (FP) Cracking diffused chromide coating/ 1200 NoMCrAlY (NP) Cracking

Each test was ceased if no coating cracking was observed at apre-determined maximum strain. For example, at the 600° F., this maximumstrain was set at 1% of cracking; for all other temperatures, themaximum strain was set at 2% of cracking.

Cracking was only observed with MCrAlY/diffused chromide coating at eachtemperature of 600° F., 800° F. and 1000° F.

None of MCrAIY/diffused chromide coating, diffused chromidecoating/MCrAIY (FP) and diffused chromide coating/MCrAIY (NP) cracked at1200° F.

Both diffused chromide coating/MCrAIY (FP) and diffused chromidecoating/MCrAIY (NP) coatings specimen did not crack at any temperatureover the range of 600° F. 1200° F. hot corrosion strain.

In conclusion, there is no indication that StC is associated with thepreparation process, e.g., DHT and peening, to MCrAlY prior to diffusedchromide coating. Furthermore, there was no observation of any LCF orstrain to crack debit with the preparation process described.

Example 5: Burner Rig Hot Corrosion Exposures

The burner rig hot corrosion tests were conducted with the coatingspecimens. Each coating specimen was exposed at 1350° F. for 980 hourswith 20 ppm sea salt in fuel and 30.5 Uh SO₂ in the gas stream. FIG. 5shows macroscopic images of each specimen at time course exposure to thetest.

After 415 hours of exposure, a thick non-protective corrosion scale wasobserved with MCrAIY and MCrAIY/diffused chromide coating. The thicknon-protective corrosion scale is associated with rapid hot corrosiondegradation. This hot corrosion degradation in those specimens occurredat approximately 415 hours and continued to worsen until the test wasterminated.

In contrast, a thin protective oxide scale was formed and maintained oneach coating of diffused chromide coating/MCrAIY (FP) and diffusedchromide coating/MCrAIY (NP) throughout the exposure time. The thinprotective oxide scale is associated with hot corrosion resistance.

FIG. 6 shows micrographic optical views of each coating specimen after980 hours of hot corrosion test at 1350° F.

Majorities of the surface of MCrAIY and MCrAIY/diffused chromide coatingcoating were defeated over 980 hours of exposure with subsequent damagesof the base substrate to maximum depths of approximately 20 mil.

In contrast, no significant damage was observed after 980 hours ofexposure for diffused chromide coating/MCrAIY (FP) and diffused chromidecoating/MCrAIY (NP).

In conclusion, the coating deposition of the diffusion coating atop thecathodic arc coating in the present invention improves the hot corrosionresistance significantly. Furthermore, there is no indication that hotcorrosion resistance is associated with the preparation process, e.g.,DHT and peening, to MCrAlY prior to diffused chromide coating.

Example 6: Preparation of an Article

A turbine blade is provided. To the turbine blade, a layer of cathodicarc coating comprising a combination of nickel, cobalt, and iron isapplied. Following application of the cathodic art coating, a diffusedchromide coating comprising a chromium rich phase is deposited on thecathodic arc layer by vapor diffusion at 1925-2000° F. to produce acoated turbine blade.

What is claimed is:
 1. A chromium-rich cathodic arc coating comprising;a MCrAlY cathodic arc coating on a substrate, wherein M is a metal alloycomprising nickel, cobalt, iron, or a combination thereof, and thecathodic arc coating comprises Si and 25 to 50% chromium by weight basedon the weight of the MCrAl; and a diffused chromide coating atop theMCrAlY cathodic arc coating, wherein the diffused chromide coating has ahigher content of chromium than the MCrAlY cathodic arc coating, thecombined thickness of the diffused chromide coating and the MCrAlYcathodic arc coating is 2.5 to 3 mil (0.0635 to 0.0762 mm) and thechromium-rich cathodic arc coating has a chromium rich phase with adepth of 0.1 to 2.5 mils (0.00254 to 0.0635 mm).
 2. The chromium-richcathodic arc coating according to claim 1, wherein the surface of thediffused chromide coating atop the MCrAlY has a chromium content no lessthan 37% by weight.
 3. The chromium-rich cathodic arc coating accordingto claim 2, wherein the MCrAlY has a chromium content between 25% and35% by weight based on the weight of the MCrAlY.
 4. The chromium-richcathodic arc coating according to claim 1, wherein the MCrAlY comprisesSi in an amount of 0.15 to 0.65% by weight based on the of the MCrAlY.5. The chromium-rich cathodic arc coating according to claim 1, whereinthe chromium rich phase has a depth of 0.2 to 1.0 mils (0.00508 to0.0254 mm).
 6. The chromium-rich cathodic arc coating according to claim1, wherein the chromium-rich cathodic arc coating is resistant to hotcorrosion at a temperature between 1200° F. and 1600° F.
 7. Thechromium-rich cathodic arc coating according to claim 1, wherein thechromium-rich cathodic arc coating is resistant to stress corrosion. 8.The chromium-rich cathodic arc coating according to claim 1, wherein thechromium-rich cathodic arc coating is resistant to low cycle fatigue. 9.The chromium-rich cathodic arc coating according to claim 1, wherein thesubstrate comprises a blade root, internal surface of turbine blade, orthe external surface of a turbine blade.
 10. An article comprising: a) asubstrate; b) a MCrAlY cathodic arc coating on the substrate, wherein Mis a metal alloy comprising nickel, cobalt, iron, or a combinationthereof and the cathodic arc coating comprises Si and 25 to 50% chromiumby weight based on the weight of the MCrAlY; and c) a diffused chromidecoating atop the MCrAlY cathodic arc coating; wherein the diffusedchromide coating atop the MCrAlY has a higher content of chromium thanthe MCrAlY cathodic arc coating, the combined thickness of the diffusedchromide coating and the MCrAlY cathodic arc coating is 2.5 to 3 mil(0.0635 to 0.0762 mm), and the diffused chromide coating and the MCrAlYcathodic arc coating have a chromium rich phase with a depth of 0.1 to2.5 mils (0.00254 to 0.0635).
 11. The article according to claim 10,wherein the substrate comprises a blade root, internal surface ofturbine blade, or the external surface of a turbine blade.
 12. Thearticle comprising according to claim 10, wherein the MCrAlY comprisesSi in the amount of 0.15 to 0.65% by weight based on the weight of theMCrAlY.
 13. The article comprising according to claim 10, whereinchromium rich phase has a depth of 0.2 to 1.0 mils (0.00508 to 0.0254mm).
 14. A method of producing a chromium-rich cathodic arc coatingcomprising steps of: a) applying a MCrAlY cathodic arc coating on asubstrate, wherein M is a metal alloy comprising nickel, cobalt, iron,or a combination thereof and the cathodic arc coating comprises Si and25 to 50% chromium by weight based on the weight of the MCrAlY; and b)applying a diffused chromide coating atop the MCrAlY cathodic arccoating at a temperature of 1800 to 2200° F.; wherein the diffusedchromide coating has higher content of chromium than the MCrAlY cathodicarc coating and the combined thickness of the diffused chromide coating,the MCrAlY cathodic arc coating is 2.5 to 3 mil (0.0635 to 0.0762 mm)and the chromium-rich cathodic arc coating has a chromium rich phasewith a depth of 0.1 to 2.5 mils (0.00254 to 0.0635 mm).
 15. The methodof claim 14, wherein the surface of the diffused chromide coating has achromium content no less than 37% by weight.
 16. The method of claim 15,wherein the MCrAlY cathodic arc coating has a chromium content ofbetween about 25% and 35% by weight based on the weight of the MCrAlY.17. The method of claim 14, wherein the chromium rich phase has athickness of 0.2 to 1.0 mils (0.00508 to 0.0254 mm).
 18. The method ofclaim 14, wherein the method of applying the diffused chromide coatingatop the MCrAlY comprises pack chromizing, slurry chromizing, vapordiffusion coating, or gas diffusion coating.